Apify server-mode (Standby) request answers 429
If no Actor run can take the request within 2 minutes, Apify answers 429; the first response must also arrive within 5 minutes. Retry after a short pause, and send one request at a time while you test.
Source: Apify documentation: Standby mode. Applies to: Server mode (one request, one answer).
The question people ask
Why does a server-mode request answer 429?